The invention relates to a magnetic switch having a cup-shaped yoke, a core, a coil, an armature and a fixed contact and a movable contact.
Magnetic switches of this kind are known, for example, from EP 0 442 311 A2, which discloses a magnetic switch in which a non-magnetic armature is moved by a permanent magnet fixedly connected thereto. During this, in the direction of movement of the armature, only an operational air gap between the permanent magnet and the core brings about the effect. Furthermore, the known magnetic switch has a small coupling surface between the permanent magnet and the core, which extends only over the inner region of the cup-shaped yoke.
The known magnetic switch has the disadvantage that the coupling surface between the permanent magnet and the core, and thus the holding force of the magnet system, is only very small relative to the structural volume of the magnetic switch. This means that the known magnetic switch is not suitable for use as a battery disconnecting switch in a motor vehicle, where on the one hand large currents of a few thousand amps have to be switched, and where on the other hand vibrations and jolts act on the magnetic switch. To achieve a greater holding force, it would be possible for special magnetic materials which generate a very strong magnetic field to be used. These have the disadvantage that they are expensive to procure and complicated to process.
The object of the present invention is therefore to provide a magnetic switch which presses the movable contact onto the fixed contact with a high holding force while having a small structural volume.
This object is achieved by a magnetic switch having a yoke that has the shape of a cup with an axis of symmetry. The cup has a base and a wall and is open to one side. Arranged in the cup is a core which has a radial core flange at its end facing the base. This radial core flange forms with the wall of the cup a marginal air gap with a width of HR. This marginal air gap has a magnetic resistance of WR. Furthermore, the cup contains a plate-shaped permanent magnet which is magnetised parallel to the axis of symmetry of the cup. The permanent magnet is arranged between the core flange and the base and is magnetically coupled thereto. Moreover, arranged in the cup is a coil whereof the coil axis is the axis of symmetry of the cup. The cup is covered by a plate-shaped armature which forms with the yoke a yoke air gap with a width of HJ and forms with the air gaps together form, connected serially one behind the other, the operational air gap, which has a magnetic resistance of WA. Moreover, there is arranged on the magnetic switch a means for lifting the armature axially away from the cup. In the closed condition of the magnetic switch, the following applies: WA less than WR.
As a result of the construction of the magnetic switch, in accordance with the invention, a magnetic switch is produced which is shielded from the outside extremely well and has only extremely low scattering losses for the magnetic fields. Because the armature forms an operational air gap with the core and the cup-shaped yoke, a coupling surface which is as large as possible is formed with a particular predetermined structural volume. The large coupling surface between the armature and the yoke and the core respectively results in a large holding force of the magnetic switch in the closed condition. Because the magnetic resistance of the operational air gap is smaller than the magnetic resistance of the marginal air gap, it is ensured that the majority of the magnetic flux of the permanent magnet is conducted through the armature in the closed condition. The system polarised by the permanent magnet moreover has the advantage that only a short switch pulse has to be sent through the coil in order to close the switch, and the coil current can be returned to zero in the closed condition. This means, in particular, that low-power operation of the magnetic switch is possible.
In order to achieve the maximum holding force of the armature, it would be desirable for the ratio WR/WA to be as large as possible. This would ensure that all of the magnetic flux of the permanent magnet is guided through the armature without losses being produced by way of the marginal air gap. A very large ratio WR/WA would, however, mean that a high counter-excitation of the coil would be required to open the switch, in order to displace the magnetic field of the permanent magnet out of the core and guide it through the marginal air gap and the wall of the cup, back onto the underside of the permanent magnet. Consequently, for the magnetic switch to have a high sensitivity to switching off, it would be desirable for the ratio WR/WA to be in the order of magnitude of one. Thus, it is particularly advantageous to set the ratio WR/WA such that both the holding force of the armature and the sensitivity of the magnetic switch to switching off are in a reasonable range. Tests have shown that this is the case if the ratio WR/WA is selected to be between 1 and 100. A ratio WR/WA=50 has been shown to be particularly advantageous.
A magnetic switch according to the invention may for example readily be produced with the following dimensions: for the cup, a radial extent of between 10 mm and 50 mm is chosen. With a size such as this, a compact construction is possible for the magnetic switch, as is specifically necessary for its use as a battery disconnecting switch in a motor vehicle. For the width of the yoke and core air gaps, purely for reasons of the superficial roughness of the underside of the armature or of the surfaces of the cup wall and the core, the following range can readily be set: 0.005 mm less than HJ less than 0.05 mm and 0.005 mm less than HK less than 0.05 mm. If a wall thickness of the cup of approximately 1 mm is chosen, moreover, then a surface for the yoke air gap with a size of approximately 100 mm2 is obtained. If the core air gap or the marginal air gap is constructed to have approximately the same order of magnitude as regards their surfaces, then to fulfil the relation mentioned above between the magnetic resistances of the operational and the marginal air gaps, the following apply: 1 mm greater than HR greater than 0.1 mm. The said dimensions of the core and yoke air gaps relate to the closed condition of the switch. With a magnetic switch constructed in accordance with these example dimensions, a holding force of approximately 30 N can be achieved. The axial extent of the switch corresponds approximately to the radial extent. A magnetic switch of this type can be switched at a switching capacity of approximately 0.7 W.
Particularly advantageous is a magnetic switch in which the yoke, permanent magnet, core, coil and armature are arranged rotationally symmetrically about the axis of symmetry. As a result of the rotationally symmetrical arrangement according to the invention of the elements of the magnetic switch about the axis of symmetry, an optimum flux concentration is achieved in the interior of the switch without magnetic field losses occurring at corners or edges. Moreover, a magnetic switch with rotational symmetry is easy to produce.
Furthermore, particularly advantageous is a magnetic switch in which the permanent magnet is of a material containing barium ferrite. Barium ferrite is a readily available, inexpensive material for a permanent magnet, which greatly simplifies production of the magnetic switch. Because of the high holding force of the magnetic switch according to the invention, expensive highly magnetised special materials for the permanent magnet can be dispensed with.
Furthermore, particularly advantageous is a magnetic switch in which the yoke and/or the core are of a material which contains magnetically soft iron. Magnetically soft iron can very readily be demagnetised, which simplifies the displacement of the flux of the permanent magnet out of the core when the magnetic switch is opened. Furthermore, magnetically soft iron has a low magnetic resistance, as a result of which it is ideal for conducting the flux of the permanent magnet to the armature.
A high holding force of the magnetic switch may be achieved in particular in that the core and the yoke are dimensioned, at least in the region of the corresponding air gaps, in cross-section such that in the closed condition and in the absence of excitation, that is to say when the coil current is 0, there is almost magnetic saturation.
Furthermore, it is particularly advantageous for the armature to be constructed in accordance with the invention with a radially varying plate thickness such that there is almost magnetic saturation in the entire armature in the closed condition of the magnetic switch. For this, it is in particular possible to make the armature thinner at the edge than in the centre, since the cross-sectional surface available for the magnetic flux increases with the distance from the axis of symmetry. The thickness of the armature can be lessened accordingly with increasing distance from the axis of symmetry. This makes it possible to reduce the mass of the armature to the absolutely minimum amount required. A minimised armature mass of this kind has the advantage that the magnetic switch is less sensitive to vibrations or shocks, since in this case it is primarily the inert mass of the armature which determines the extent to which the armature can respond to rapid accelerations.
Furthermore, the magnetic switch may be constructed to be particularly compact if, according to the invention, a hole running along the axis of symmetry is provided in the yoke, the permanent magnet, the core and the armature, and through this hole runs a pin. This pin is secured to the armature at one end, and at the other end is coupled to a movable contact which makes contact with a first fixed contact in the closed condition of the magnetic switch. With the aid of the movable contact and the fixed contact, the circuit of a motor vehicle, for example, may be switched.
Moreover, particularly advantageous is a switch in which the movable contact is a flange-shaped contact disc which is arranged symmetrically with respect to the axis of symmetry and has a central hole. In this case, the pin is secured in the hole in the armature by means of flange-shaped end portions and is suspended in the contact disc from a collar projecting into the contact disc hole. The contact disc makes contact with a first and a second fixed contact in the closed condition. Arranged in the hole in the permanent magnet is a pressure spring contact supported against the armature and the contact disc. The end of the pin remote from the armature is pre-tensioned in the direction of the armature via a rocker by means of a tension or pressure spring. The rocker according to the invention, which is coupled to the pin, is a simple means of raising the armature axially away from the cup. The pressure spring contact supported against the armature and the contact disc has the advantage that the holding force of the armature can be transmitted over a certain distance, regardless of the spacing between the fixed contacts and the base of the cup.
Here, it is particularly advantageous to select the length of the pin, in accordance with the invention, such that when the magnetic switch opens, the lower pin flange moves by an overtravel distance Hu before it engages with the contact disc. This overtravel has the advantage that, when the magnetic switch opens, first the armature is greatly accelerated, together with the pin, as a result of the pressure spring contact, and only once the lower pin flange has engaged with the collar are the contacts pulled open by the forces of inertia thereby gained. The overtravel distance is preferably set in a range of between 0.5 mm and 1 mm.
Furthermore, particularly advantageous is a magnetic switch in which the pin is suspended in a spring contact secured to the second fixed contact and bearing the movable contact. The arrangement according to the invention, of a spring contact on the second fixed contact, has the advantage that it is possible to dispense with the pressure spring contact inside the cup. This means that the construction of the spring contact is no longer determined by the dimensions of the cup, but that it can be shaped and arranged outside the cup in almost any desired way.
For the spring contact, a possible material is for example spring steel or a nickel/iron alloy. Although these materials have a very good spring action, they are not suitable for conducting electrical current from the second fixed contact to the movable contact, because of their poor conductivity. Furthermore, it is particularly advantageous for the movable contact to be electrically connected to the second fixed contact by means of a litz wire. This litz wire may for example be of copper, which conducts electrical current very well.
So that the magnetic switch can be operated even if the circuit fails, it is particularly advantageous to arrange on the rocker an actuating element which can be used to open the switch manually. This actuating element moreover has the advantage that it can be arranged such that its position can be visually recognised on the switch from the outside. This means that the means of manually opening the switch can be used as an indicator of the condition of the switch at the same time.